3. THE RPC MODEL

The ONC RPC protocol is based on the remote procedure call model, which is similar to the local procedure call model. In the local case, the caller places arguments to a procedure in some well- specified location (such as a register window). It then transfers control to the procedure, and eventually regains control. At that point, the results of the procedure are extracted from the well- specified location, and the caller continues execution.

The remote procedure call model is similar. One thread of control logically winds through two processes: the caller's process, and a server's process. The caller process first sends a call message to the server process and waits (blocks) for a reply message. The call message includes the procedure's parameters, and the reply message includes the procedure's results. Once the reply message is received, the results of the procedure are extracted, and caller's execution is resumed.

On the server side, a process is dormant awaiting the arrival of a call message. When one arrives, the server process extracts the procedure's parameters, computes the results, sends a reply message, and then awaits the next call message.

In this model, only one of the two processes is active at any given time. However, this model is only given as an example. The ONC RPC protocol makes no restrictions on the concurrency model implemented, and others are possible. For example, an implementation may choose to have RPC calls be asynchronous, so that the client may do useful work while waiting for the reply from the server. Another possibility is to have the server create a separate task to process an incoming call, so that the original server can be free to receive other requests.

There are a few important ways in which remote procedure calls differ from local procedure calls:

1. Error handling: failures of the remote server or network must be handled when using remote procedure calls.

2. Global variables and side-effects: since the server does not have access to the client's address space, hidden arguments cannot be passed as global variables or returned as side effects.

3. Performance: remote procedures usually operate one or more orders of magnitude slower than local procedure calls.

4. Authentication: since remote procedure calls can be transported over unsecured networks, authentication may be necessary. Authentication prevents one entity from masquerading as some other entity.

The conclusion is that even though there are tools to automatically generate client and server libraries for a given service, protocols must still be designed carefully.